Optimizing grafting thickness of zwitterionic sulfobetaine polymer on cross-linked polyethylene surface to reduce friction coefficient

Abstract

The purpose of this study involves reducing the friction coefficient of cross-linked polyethylene (CLPE) by optimizing the grafting thickness of zwitterionic poly(2-(methacryloyloxy)ethyl dimethyl-(3-sulfopropyl)ammonium hydroxide) (PMEDSAH) via a photo-induced free radical polymerization process. Cross-sectional TEM images and XPS spectra indicated that the PMEDSAH layers were successfully grafted on the CLPE. The grafting thickness increased when UV-irradiation time, UV-irradiation energy, and MEDSAH monomer concentration increased. The contact angle data indicated that the PMEDSAH layers successfully imparted hydrophilicity (below 20°) on the CLPE surface. The friction coefficient test and the resulting 3-D confocal images confirmed a significant decrease in the friction coefficient with increases in the grafting thickness. A minimum grafting thickness of 200 nm PMEDSAH was required to minimize the friction coefficient in an aqueous medium. However, a PMEDSAH-grafting thickness of approximately 40 nm was sufficient to reveal the minimized friction coefficient in simulated body fluids (ionic buffer solution) due to the salt-induced hydration effect. The results defined the minimum required thickness of PMEDSAH grafting to minimize the friction coefficient on CLPE surface that can potentially be applied for artificial hip joints.